Numerous tension gauges exist in the prior art. However, the vast majority of these gauges only provide the ability to determine the average total tension in an elastic material without providing any information relating to the magnitude or direction of the principal tensions in the material. In addition, many of these tension gauges measure the elastic material distortion in order to derive tension. As a result, the tension measurements obtained from these gauges are subject to the tensile characteristics of the particular elastic material as well as expansions and contractions of the elastic material due to temperature.
Typical tension gauges are electrical or mechanical strain gauges which measure the elastic material distortion in order to derive tension. These gauge measurements generally give only sufficient information to determine tension in one direction or the total tension in an elastic material. While some embodiments of available electrical strain gauges employ multiple gauges, such as in the stress-gauge rosette, in order to obtain additional information about an elastic material, such measurements still require knowledge of specific elastic material characteristics for calculations of the desired information.
An example of a stress gauge for films is disclosed in U.S. Pat. No. 3,040,595 (Church, et al) issued on June 26, 1962. This gauge measures resonance under induced vibration to determine stresses in a film. While the patent discloses that it is sensitive to the magnitude and direction of stresses in the films, it is badly affected by bi-directional stresses and the patent gives no indication that it can be properly applied to determine a stress field. Further, this gauge is somewhat sensitive to the mass properties of the film being measured.
U.S. Pat. No. 4,590,808 (Lightfoot, et al) issued on May 27, 1986, discloses a device for measuring the average tension on tennis racket strings. In general, the device distorts the tennis racket strings out of a tension plane and measures the resulting force and/or deflection of the tennis racket strings. In this manner, the average tension of the tennis racket strings is obtained.
U.S. Pat. No. 4,103,546 also discloses a device for measuring the average tension in tennis racket strings. Again, the device distorts the tennis racket out of a tension plane and measures the resulting force and/or deflection of tennis racket strings to obtain the average tension on the strings.
U.S. Pat. No. 4,756,199 (Merritt) issued on July 12, 1988, discloses a device for measuring the tension normal to a displaced stocking held under sizing tension. The gauge deflects the stocking out of a tension plane and measures the resulting force to obtain the average tension on the stocking at a particular location.
British Pat. No. 115,338 issued on May 9, 1918, discloses an apparatus for measuring the tension of stretched fabrics. The device includes a ring over which a fabric material is retained by tension. A plunger contacts the center of the fabric material and is pushed upwardly by the tensile force exerted when stretching the fabric material over the ring frame. The movement of the plunger is measured to determine the average tension in the fabric material.
U.S. Pat. No. 4,587,855 (Yamada, et al) issued on May 13, 1986, discloses a tension meter of the type used to measure the tension of an elongated object such as string or rope. This tension gauge operates along a single axis and is only useful for rope or wire.
U.S. Pat. No. 4,048,850 (Ramberg, et al) issued on Sept. 20, 1977, discloses a mechanism for gauging the expansion of a trawl net. This device measures net tension in one direction directly by taking-up load and therefore suffers from many sources of error caused by local stress distributors.
U.S. Pat. No. 4,674,341 (Koenig) issued on June 23, 1987, discloses a web tension transducer apparatus for measuring web tension along a single axis. The device employs roller force to measure web tension, and, thus, is only capable of measuring the tension along a single axis when the material is forced against a web.
U.S. Pat. No. 4,024,755 (Queben) issued on May 24, 1977, discloses a sheet-flatness detector which employs a deflector roll for measuring and checking the flatness of sheet metal under tension. The device does not measure the tension of the sheet metal.
From the foregoing discussion, it is apparent that there is a need in the art for a tension gauge which is capable of providing specific information about the magnitude and direction of the principal tensions in an elastic material. Further, there is a need in the art for a tension gauge and method for determining the principal tensions for nets of crossed filaments wherein the direction of the tensions is desired as well as the magnitude.